HVDC transmission system with bidirectionally controlled thyristors

ABSTRACT

A plant for transmitting electric power having a direct voltage network (18) for High Voltage Direct Current and at least one pole conductor interconnecting two stations (21, 22). In each station (21, 22), a bidirectionally controlled thyristor (BCT) conducts current in both directions as rectifying semiconductor components. A control unit (35) is adapted to control the current direction through the bidirectionally controlled thyristors so as to choose that the earth electrode (33, 34) of each station is always either anode or cathode.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a plant for transmitting electric powercomprising a direct voltage network for High Voltage Direct Current(HVDC) having at least one pole conductor extending between andinterconnecting at least two stations with, for the pole conductor, aline commutated valve with thyristors as rectifying semiconductorcomponents for transmitting electric power between the direct voltagenetwork and alternating voltage networks connected to the stations, saidpole conductor being at each station connected to an earth electrode.

The invention is in particular directed to the case of bipolar directvoltage networks having two pole conductors, and as a consequencethereof that case will be discussed more thoroughly hereinafter, butthis is only done for illuminating the invention and the problems to besolved thereby and is not at all to be interpreted as limiting theinvention, but this relates also to the case of a plant having only onepole conductor.

A plant already known having a direct voltage network 1 with two poleconductors 2, 3 extending between two stations 4, 5 and which are ateach station provided with a valve each 6, 7 and 8, 9, respectively, fortransmitting electric power between an alternating voltage network 14and 15, respectively, connected to each station through transformers 10,11 and 12, 13, respectively, is illustrated in FIG. 1. The two poleconductors are at each station connected to a common earth electrode 16and 17, respectively. The valves are line commutated and haveconventional thyristors with only one conducting direction as rectifyingsemiconductor components, which means that the current through eachvalve and by that in a given pole conductor only may assume a determineddirection, so that the direction of the feeding of power between thestations only may be changed by changing the polarity of the poleconductors. Would now one of the pole conductors drop out, the plantwould then become a plant having a unipolar direct voltage network withone pole conductor and an earth return circuit, in which in the case ofthe drop out of the pole conductor 3 the earth electrode 16 will becomeanode and the earth electrode 17 cathode, while on drop out of the poleconductor 2 the earth electrode 16 will become cathode and the earthelectrode 17 anode. This means that the two earth electrodes have to beable to function both as anodes and cathodes should a plant be able tofunction after drop outs, since the earth electrodes on drop out of onepole conductor may assume one or the other type depending upon which ofthe poles drops out. However, the earth electrode is in certain casesdepending upon the character of the ground not at all allowed to assumean optional polarity, which means that it will be necessary to eithermove the earth electrode in question or restrict the operation to onlycertain configurations. An earth electrode located in clay ground is forexample not allowed to function as anode, since there is then a risk ofexplosions as a consequence of osmosis and gas formation, while itdoesn't matter if a cathode is arranged in clay ground. However, anarrangement of an electrode in such a ground may not be considered forthe plant already known, since it has to be able to assume an optionalpolarity. This means that it is necessary to locate the electrodes onsuch places that unnecessarily long lines or conductors are required.Furthermore, certain materials may not be used for the earth electrodes,such as copper, which is only suited as material for a cathode.

It may also be mentioned that in the case of a plant having from startonly one pole conductor and an earth return circuit, in whichaccordingly each earth electrode is determined to always be eithercathode or anode, would it be possible to later on discover that itwould as a consequence of the ground conditions prevailing beadvantageous that the earth electrodes had the opposite polarity, but itwould then be necessary to move the earth electrodes to other places orrebuild the valves, so that the polarity of the electrodes would bechanged, which would generate very high costs and delay the putting intooperation of the plant, which in some cases would result in tremendoushigh delay penalties for the deliverer.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plant of the typedefined in the introduction, which to a far extent find a remedy to theinconveniences mentioned above of already known such plants.

This object is according to the invention obtained by the fact that insuch a plant at least a valve of one pole conductor in each station hasbidirectionally controlled thyristors (BCT) with ability to conductcurrent in both directions therethrough as rectifying semiconductorcomponents, and the plant comprises a control unit adapted to controlthe current direction through the bidirectionally controlled thyristorsso as to make a choice that the earth electrode of each station shall beeither anode or cathode.

This means that in the case of two pole conductors the current directionin the pole conductors of the direct voltage network may always be sothat one earth electrode functions as anode and the other as cathode.Accordingly, it will be possible to always ensure that on drop out ofone pole conductor the current in the remaining pole conductor flowsfrom a certain of the stations to the other station, the earth electrodeof which will always be anode.

The invention means in the case of only one pole conductor that it ispossible when it is discovered after a certain operation time of theplant that the wrong earth electrode has been selected as anode andcathode, respectively, to simply reverse the current direction betweenthe stations for carrying out a change of electrode type of eachstation.

According to a preferred embodiment of the invention the direct voltagenetwork is bipolar with two said pole conductors, each of which is ateach station provided with one said valve and the pole conductors are ateach station in common connected to an earth electrode, and at least onepole conductor at each station has bidirectionally controlled thyristors(BCT) with ability to conduct current in both directions therethrough asrectifying semiconductor components.

The invention is especially applicable to the bipolar case, and it meansa possibility to ensure that the earth electrode of a given station onlyhas to function as either anode or cathode independently of which of thepole conductors of the direct voltage network has possibly dropped out,so that it is not necessary to consider the character of the groundwhere the earth electrode is arranged that much as if it could assumeboth polarities, and the choice of material for the electrode willneither be that critical. Copper may for example be used when it is surethat the electrode will always be a cathode.

According to another preferred embodiment of the invention, whichconstitutes a further development of the embodiment last mentioned, thecontrol unit is adapted to control the valves of the plant so that ondrop out of any of the pole conductors of the direct voltage network thecurrent of the remaining pole conductor always flows in the samedirection between the stations so that the earth electrode of therespective station will always be either anode or cathode. It is by thisensured that the advantages associated with the arrangement of thevalves with bidirectionally controlled thyristors are utilised to anoptimum.

According to a further preferred embodiment of the invention only one,first pole conductor has bidirectionally controlled thyristors in thevalves thereof, while the valves of the second pole conductor areprovided with thyristors having only one conducting direction, and theconducting direction of the thyristors in the valves of the second poleconductor is such that on drop out of the first pole conductor thecurrent direction through the valves of the second pole conductor meansthat the earth electrodes of the stations assume a desired anode orcathode property. Thus, the current direction through the remaining poleconductor will in the case of a drop out of the pole conductor havingvalves provided with bidirectionally controlled thyristors be such thatthe earth electrodes receive the correct polarity, while on drop out of"the wrong" pole conductor, i.e. the one having valves with thyristorswith only one conducting direction, the current direction has to bechanged through the valves of the remaining pole conductor, which mayeasily be done thanks to the thyristors present therein through atemporary gradually reduction of the current through the pole conductorto zero, whereupon the current is then gradually increased again in theopposite direction, so that the polarity of the earth electrode ismaintained.

Further advantages as well as advantageous features of the inventionappear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWING

With reference to the appended drawing, below follows a description of apreferred embodiment of the invention cited as an example.

In the drawing:

FIG. 1 is a very simplified circuit diagram illustrating a plant fortransmitting electric power between two stations through a directvoltage network for High Voltage Direct Current (HVDC) according to theprior art, and

FIG. 2 illustrates a plant according to the present invention in a viewcorresponding to FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The structure of a plant according to a preferred embodiment of theinvention for transmitting electric power, which comprises a directvoltage network 18 for High Voltage Direct Current (HVDC=High VoltageDirect Current) with two pole conductors 19, 20, which interconnects twostations 21, 22, is very schematically and simplified illustrated inFIG. 2. Each pole conductor has at each station a valve 23-26 each,which is connected to an alternating voltage network 27, 28 through atransformer 29-32. The pole conductors 19, 20, or expressed more exactlythe two valves, are in each station connected in common to an earthelectrode 33, 34.

The high voltage network 18 has typically a potential difference of10-500 kV with respect to earth.

The respective valve is in a conventional way formed by for exampleso-called 12-pulse bridges, in which a number of rectifyingsemiconductor components in the form of thyristors are connected inseries, since they in the turned-off state each can normally only hold1-10 kV and the considerably higher voltage across the valve has to bedistributed among the different thyristors.

The thyristors are in the valves 23 and 25 belonging to the one poleconductor 19 of conventional type, i.e. they and by this these valvesmay only conduct current in one direction therethrough, i.e. from thestation 22 to the station 21.

However, the valves 24 and 26 of the second pole conductor 20 aredesigned in another way, by the fact that said thyristors are replacedby so-called BCTs (Bidirectionally Controlled Thyristors), i.e.bidirectionally controlled thyristors, which are made of two halves ofone and the same disc connected in antiparallel to each other, so thatthrough a unit 35 being for the sake of simplicity only indicated forthe valve 24 these thyristors belonging to the valve may be controlledfor selecting the current direction through the valve completelyindependently of the polarity existing of the pole conductor 20. Thebidirectionally controlled thyristors are thyristors of the typedescribed in for example DE 44 39 012 A1 and are sometimes calledtwo-direction thyristors.

In normal operation of the plant the current flows from the station 22to the station 21 through the pole conductor 19 and then back to thestation 22 through the pole conductor 20. By choosing positive polarityfor the pole conductor 19 and negative for the pole conductor 20 powermay be fed from the station 22 to the station 21 on the direct voltagenetwork, while the opposite feeding direction of power may be obtainedthrough changing the polarity of the pole conductors.

It is decided in the present case that the earth electrode 33 shallalways function as anode and the earth electrode 34 as cathode. Wouldnow the pole conductor 20 drop out, so that a change to unipolaroperation with earth return circuit takes place, the current would thenflow through the pole conductor 19 from the station 22 to the station 21and then back through the anode 33 to the cathode 34. However, would thepole conductor 19 drop out, the current in the pole conductor 20 hasthen for the moment the wrong direction, and as a consequence thereofthe control unit 35 controls the two valves 24 and 26 to graduallyreduce the current to zero, whereupon a current direction change orderis given and the current is after that gradually increased to thedesired level, so that it will flow from the station 22 to the station21 in the pole conductor 20 and by that the earth electrode 33 will beanode and the earth electrode 34 cathode.

The invention is of course not in any way restricted to the preferredembodiment described above, but many possibilities to modificationsthereof would be apparent to a man skilled in the art without departingfrom the basic idea of the invention such as it appears from the claims.

For example in a bipolar plant all the valves could be provided withbidirectionally controlled thyristors as rectifying semiconductorcomponents, although it is not necessary for ensuring that the earthelectrode polarity is always the same.

It is also conceivable that the plant has more than two stations.

The claim definition that the pole conductor is at each stationconnected to an earth electrode is not to interpreted as a requirementthat it is directly connected thereto, but it is instead so that atleast a valve of the pole conductor is connected therebetween.

What is claimed is:
 1. A plant for transmitting electric powercomprising a direct voltage network for High Voltage Direct Current(HVDC) having at least one pole conductor extending between andinterconnecting at least two stations with, for the pole conductor, aline commutated valve with thyristors as rectifying semiconductorcomponents for transmitting electric power between the direct voltagenetwork and alternating voltage networks connected to the stations, saidpole conductor being at each station connected to an earth electrode,wherein a valve of at least one pole conductor in each station includesbidirectionally controlled thyristors (BCT) for conducting current inboth directions therethrough as rectifying semiconductor components, andthe plant comprises a control unit for controlling the current directionthrough the bidirectionally controlled thyristors so as to make a choicethat the earth electrode of each station shall be either anode orcathode.
 2. A plant according to claim 1, in which the direct voltagenetwork is bipolar with two said conductors, each of which at eachstation being provided with one said valve and the pole conductors beingcommonly connected at each station to an earth electrode, wherein thevalve of at least one pole conductor at each station has bidirectionallycontrolled thyristors (BCT) for conducting current in both directionstherethrough as rectifying semiconductor components.
 3. A plantaccording to claim 2, wherein the control unit is adapted to control thevalves of the plant so that on drop out of any of the pole conductors ofthe direct voltage network the current of the remaining pole conductoralways closes in the same direction between the stations so that anearth electrode of the respective station will always be either anode orcathode.
 4. A plant according to claim 3, wherein only a first of thepole conductors has bidirectionally controlled thyristors in the valvesthereof, while the valves of the second pole conductor are provided withthyristors having only one conducting direction, and the conductingdirection of the thyristors in the valves of the second pole conductoris such that on drop out of the first pole conductor the currentdirection through the valves of the second pole conductor means that theearth electrodes of the stations assume a desired anode or cathodeproperty.
 5. A plant according to claim 1, comprising two stations.